The Macintosh serial modem port uses an 8-pin mini-DIN connector (introduced with the Macintosh Plus) and implements EIA RS-422 balanced differential signalling via a Zilog Z8530 Serial Communications Controller. Pin 1 (HSKo) is driven from the SCC /DTR output and serves as the output handshake line; in this cable it is wired to both RTS (DB-25 pin 4) and DTR (DB-25 pin 20) on the modem end to satisfy modems that require either signal for initialisation. The Mac’s signal ground and the uninverted receive line (RxD+, pin 8) are combined to provide a clean ground reference to the modem. Maximum asynchronous data rate via the Mac Toolbox serial driver is 57,600 baud.
8 pin mini-DIN male connector to the Computer
25 pin D-SUB male connector to the modem
| Mac | Dir | Modem | ||
|---|---|---|---|---|
| HSKo | 1 | OUT | 4+20 | RTS+DTR |
| HSKi | 2 | IN | 5 | CTS |
| TxD- | 3 | OUT | 2 | TxD |
| RxD- | 5 | IN | 3 | RxD |
| GND+RxD+ | 4+8 | - | 7 | GND |
| GPi | 7 | IN | 8 | DCD |
Signals
- HSKo — Output handshake; driven inverted from the SCC /DTR output. Wired to both RTS and DTR on the DB-25 modem connector. Voh = +3.6 V, Vol = −3.6 V.
- HSKi — Input handshake; connected to SCC /CTS and /TRxC. Also accepts an external clock signal. Vih = +0.2 V, Vil = −0.2 V.
- TxD− — Transmit data, inverted (RS-422 negative leg). Tri-stated when /RTS is not asserted.
- RxD− — Receive data, inverted (RS-422 negative leg); received inverted at SCC RxD.
- GND+RxD+ — Signal ground (pin 4, connected to logic and chassis ground) combined with RxD+ (pin 8, uninverted receive data); together they provide the modem ground reference.
- GPi — General-purpose input; received inverted at SCC DCD. Wired to modem DCD (DB-25 pin 8). Not connected on the Macintosh Plus.
Notes
- This cable is for DTE-to-DCE connections (computer to modem) where the modem requires an active DTR signal.
- RS-422 differential signalling requires that both the inverting and non-inverting lines of each pair be terminated correctly; combining GND and RxD+ at the modem ground pin is the standard Apple-recommended practice for this cable type.
- The mini-DIN-8 connector is keyed and cannot be inserted incorrectly; ensure the notch aligns with the port housing.
- Port A (the modem port) has higher interrupt priority than port B (the printer port) on all compact Macs.
- TxD− (pin 3) and TxD+ (pin 6) are both tri-stated when the SCC /RTS output is not asserted; pin 6 is not connected in this cable.
References
- Guide to the Macintosh Family Hardware, Second Edition — Apple Computer, Inc. / Addison-Wesley, 1990
- Apple Service Technical Procedures: Peripheral Interface Guide, PN 072-0213F — Apple Computer, Inc., January 1991